This invention relates to filtration and more particularly to membrane filter cartridges and to components and methods for making them.
Membrane filters of various polymeric materials are known and are generally thin porous structures having porosities between about 50-80% by volume. They are relatively fragile and are commonly used with various types of mechanical support or reinforcement. Flow rates of liquids through such membranes per unit of area are a function of pore size, flow rates decreasing with decreasing pore size. To obtain high flow rates through filters with fine pores, for example below about one micron, relatively large filter areas are needed. Such areas have therefore been provided by using large individual filters or by using a number of smaller individual filters in parallel. For use in critical applications such as sterilization, such membranes and their supporting apparatus must be free of leaks or defects capable of passing small particles or organisms.
Numbers of small filters have heretofore been hand-assembled for parallel flow with supporting plates and associated apparatus, then tested, and, if necessary, sterilized, often at the user's site at considerable cost and inconvenience. The operations must be repeated if the hand assembly fails the necessary tests. The mechanical parts are generally cleaned and re-used, only the filters being replaced. One assembly heretofore provided in disposable plastic has also been mechanically secured with relatively moveable parts.
Individual membrane filters of large area have been supported flat or cylindrically, or have been pleated for disposition in compact housings. Holders for flat membranes are large, for a given filter area, are usually not disposable, and also require disassembly, cleaning, reassembly and testing with each change of filter. Pleating of fragile membranes creates stress concentrations at the folds, permits flexing of the fragile membranes in use, normally requires interleaving flow screens on one or both of the upstream and downstream sides, requires potting and adhesives to seal the ends and overlapping seams, and requires considerable hand operation. Because of concern for possible failures at the folds, seams, or ends, a separate flat final filter is sometimes used in series with pleated cartridges for added assurance in critical applications, for example, in sterilizing pharmaceuticals and intravenous fluids. In addition, the use of a number of different materials in pleated cartridge construction increases the sources for extractibles into the filtrate.